It's a test I have aways declined with all my pregnancies ,it not a definite yes or no it's an assessment of risk factors , neck measurement , nasal bone , eye measurements , also the mother's risk factors are taken into considation her age and health . Then you get your risk calculation .
Even if you decline the test these measurements are carried out and you would know if something showed up fortunatley mine never have but it's all there in your notes for you to read if you wished and then you could decide to go privately if you wanted anyway .
It is personal choice at the end of the day , good luck with what ever you decide to do .xx